Echo character of the East Brazilian continental margin and its relationship to sedimentary processes

The nature and regional distributions of various types of bottom echoes recorded on 3.5-kHz echograms from the East Brazilian continental margin (8–30°S) provide valuable information about sedimentary processes which have been active on a regional scale. The ten types of echoes observed fall into two major classes: distinct and indistinct. Indistinct echoes have two sub-classes; prolonged and hyperbolic. A qualitative correlation is observed between three types of distinct and indistinct-prolonged echoes and the relative abundance of coarse, bedded sediment (silt, sand, gravel) in piston cores. Regions returning distinct echoes with continuous parallel sub-bottoms contain little or no coarse sediment; regions returning indistinct very prolonged echoes with no sub-bottoms contain very high concentrations of coarse sediment; and regions returning indistinct semiprolonged echoes with intermittent sub-bottoms contain moderate or intermediate amounts of coarse sediment. Thus the regional distributions of these three echo types reflect the dispersal of coarse terrigenous sediment throughout the region. High concentrations of coarse sediment are restricted to relatively small areas which are generally proximal to large deep-sea channels, whereas very low concentrations occur in distal regions such as the lowermost continental rise and adjacent abyssal plain. Moderate concentrations of coarse sediment occur throughout most of the continental rise. Five of the six types of hyperbolic echoes observed are reflected from erosional/depositional bed forms. Although some of these bed forms (especially on the upper continental rise) have probably been produced by gravity-controlled mass flows (turbidity currents, slumps, etc.) the fact that the most extensive and widespread regions of hyperbolic echoes occur in distal regions beneath the present axis of flow of the Antarctic Bottom Water suggests that most of these bed forms are the result of sediment reworking by the contour-following bottom currents of this water mass.